This invention relates to methods for the production of disease suppressive compost, as well as container media amended with such compost, and microorganism cultures for use in such methods. More particularly, this invention relates to methods for the production of disease suppressive compost in which one or more microorganisms antagonistic to plant pathogens are added to the compost, the timing of the addition of the microorganisms to the compost preferably being carefully controlled to produce the greatest suppression of the pathogens.
A variety of microorganisms, especially members of the genera Trichoderma and Pseudomonas, are antagonists to plant pathogens such as Rhizoctonia solani, Pythium ultimum and others. Furthermore, it is known that suppression of such pathogens in commercial crops such as radishes may be achieved by using on such crops a compost containing relatively high concentrations of the antagonistic microorganisms. However, hitherto it has not been possible to produce, in a reproducible manner, composts containing sufficient numbers of the antagonistic microorganisms. Composts produced in windows are typically recolonized, after peak heating, by large numbers of different microorganisms and, although some of these microorganisms are of antagonistic varieties, hitherto there has been no way of ensuring that the antagonistic microorganisms do achieve sufficient population densities to render the resulting compost suppressive to the plant pathogens.
Container media amended with composted hardwood tree bark (CHB) suppress Rhizoctonia damping-off (Rdo) (Nelson and Hoitink, 1982, Stephens, et al., 1981). Suppressiveness of CHB container media to Rdo varies with compost age and those amended with mature CHB (&gt;11 weeks of composting) are most suppressive (Nelson and Hoitink, 1983). The suppressive effect lasts for at least 2 yrs. Container media with Canadian sphagnum peat as the sole organic component are conducive, although a source of light sphagnum peat has been described that may be suppressive for up to 7 weeks after planting.
Suppressiveness of CHB container media is microbial in nature (Nelson and Hoitink, 1982, 1983). Previous work with fungi isolated from suppressive and conducive CHB container media has shown that a relationship exists between population levels of certain fungi and Rdo suppression (Kuter et al, 1983). Isolates of Trichoderma hamatum (Bonord) Bain, aggr. and T. harzianum Rifai, the most abundant fungal taxa isolated from suppressive CHB container media (Kuter et al), are also the most efficacious fungi in inducing suppression to Rdo (Nelson et al 1983). The efficacy of T. harzianum is affected by the maturity level of CHB used in preparation of the container medium (Nelson et al 1983).
Container media amended with composts prepared from tree bark suppress a variety of soil-borne plant diseases including those caused by Pythium spp. (Plant and Soil 39: 591-602 (1973) Plant Dis. 64: 142-147 (1986), Annu. Rev. Phytopathol. 24: 93-114 (1986), Phytopathology 73: 274-278 (1983)). Container media that contain bark have largely replaced peat container media for production of nursery stock and some floral crops in the United States, Australia and European countries (Biocycle 25: 42-45 (1982)). Recycling of organic wastes into composts increasingly is chosen as a least objectionable procedure in dealing with waste problems. An increasing number of composts types, therefore, is becoming available to the ornamentals industry. The impact of composts on the soil microflora, plant pathogens and plant diseases is not well understood. More information is needed before effects of such amendments of container media are predictable.
The need for some method to produce sufficient populations of antagonistic microorganisms in compost, and also container media amended by the compost, is exacerbated by recent changes in commercial manufacture of compost. Until recently, most compost has been prepared in windrows or aerated piles exposed to weather. Recently, however, controlled processes using aerated silos or aerated tanks have been developed; in such processes, the material to be composted is placed in a substantially enclosed container through which air is forced in a controllable manner. Such processes allow better control over composting conditions and thus produce a more uniform product. Unfortunately, composts produced under such controlled conditions are normally recolonized after peak heating by a less diverse microflora than compost produced in windrows (Phytopathology 73: 1450-1456 (1983)), so that compost produced under such controlled conditions is less likely to be suppressive to plant pathogens.
Furthermore, with the increased attention being paid in recent years to reducing pollution of the environment, and more specifically pollution of surface waters, a large number of additional and/or refurbished sewage plants have been constructed. Such plants produce large quantities of sludge which must be disposed of, and the most convenient way to dispose of such sludge is composting followed by use of the composted sludge on agricultural land. However, the techniques used for composting sewage sludge in modern plants are not conducive to colonization of the sludge by antagonistic microorganisms which will render it suppressive to plant pathogens such as Rhizoctonia and Pythium. Consequently, if such composted sewage sludge is to be rendered suppressive to such pathogens, as is highly desirable where the composted sewage sludge is to be used for agricultural purposes, artificial inoculation of the composted sewage sludge with antagonistic microorganisms is necessary.
Finally, in view of the significant losses caused to various commercial crops by Rhizoctonia and Pythium, it is highly desirable that compost and container media should be suppressive to both these pathogens, and prior art methods do not yield composts and container media which are reliably suppressive to both pathogens.
It will thus be seen that there is a need for a method of reproducibly producing a compost suppressive to plant pathogens, and specifically a need for a method of reproducibly producing a compost and container media suppressive to both Rhizoctonia and Pythium. Furthermore, the method to be used for producing such suppressive compost should be usable in controlled composting processes, particularly those now used for composting sewage sludge, as well as in windrow or aerated pile composting processes. This invention seeks to provide methods for producing such suppressive composts and container media, and a microorganism culture for use in such methods.